Recommended Fender BF Princeton Reverb Clone Mods

[orting]

5u4 and large caps

"Depending on the rectifier used, this might be a little large. If you're using a GZ34/5AR4 rectifier you'll be OK, but if you use a 5U4GB 40uf should be the max capacitance used. Are you using a multi-section cannister type cap like used on the PR, or individual caps as used on the larger Fenders? You can get a 40/20/20/20 cap from Weber and from Antique Electronics"

Help me out here, why is it not good to use cap values of greater than 40uf with a 5u4 rectifier? Is this only an issue when the cap is in series to ground with the connections to component sections of the amp or when used off of B+ to ground prior to connections to component sections of the amp (as found in later versions of say, the Bandmaster amps using diode rectifiers)?

Thanks in advance!

[J Martin]

40uf is the book value for a 5U4GB, in reality you can go a little higher. Too high and you risk burning out the rectifier. There is no reason to go any higher for your project as that will be adequate filtering. If you don't already have a tube manual, you should get one. Repros are available from places like Antique Electronics Supply.

[TD_Madden]

I'm using a repro tweed deluxe OT in my PR......sounds just fine with an Eminence CopperHead.

Ordered a replacement baffle with offset 12" cutout from Mojo as well....just to play with y'know!

[PeaveyBandit]

Can someone proof my MODS??

Hello once again. I'm about to get all the few remaining parts in hand and I believe I have a completed Princeton Reverb mod plan from the first build.

I'm installing all 3 of the stokes, paulc and rick e, mods. outlined in this PDF file I am attaching.

I'm also attaching my modified layout drawing with grid resistors and also a bias circuit I liked from I believe the Hoffman site under the princeton adj bias mods.

I'm concerned also because I'm going to a 5AR4 rectifier as well. The hoffman adju bias mod calls for that new cap. And I'm also kinda curious how I will preadjust that 10KL pot and resistor before the first power up. The way he moves the resistor so it's no longer parallel with the cap confuses me a little.

Any insight GREATLY appreciated. I just want to see if my plan is perhaps taking too many steps at once or maybe some piece of it improper or not a good idea.

I may add the lower value bypass cap recommendation later but I want to see how the stock is first. It's such a small cab I might not want to reduce the bass.

[Rick Erickson]

The 6V6 Screens of a Princeton are already pulling current together through a 1K resistor in the power supply. Adding an additional 470 Ohms in series with each tube will limit screen current even further. I have not personally seen a need to do this, but modern 6V6's might last a little longer with the reduced screen current.

RE

[[ula58]

Does the "Rick E" mod (changing an 18k power supply resistor from 18k to 4.7k) place too much stress (due to high voltage) on the PI 12AX7 tube? Will this cause reliability issues?

[[ula58]

anyone know about the rick e mod effect on AX7 reliabilty due to higher plate voltage?

[PeaveyBandit]

anyone know about the rick e mod effect on AX7 reliabilty due to higher plate voltage?

I'm just commenting that I don't have the 18K resistor on my amp lowered to the 4.7K of the RickE mod but I DO have everything else the same as the document that outlines ALL 3 mods in one installation. So while I can't comment on your exact question I can say that I'm using a 15K in that power supply position with great tone, power and results. I'm probably not going to lower it because the amp, speaker and cab seem to be operating with everything it should have short of becoming a mesa boogie style nuclear brick. At any higher gain this amp would probably stop having the dynamics that I'm digging so much. It's like a Deluxe with a hair trigger response as I have it. Hope that input is helpful in some way.

[Amp Kat]

Speaking of that Rick I play a BF Princeton reverb all the time and have to say the difference with a good set of NOS power and rectifier tubes and a wrath of Telefunkens across is a seriously huge major improvement over the current production so, it may cost you Peavey B. but well worth IMO the excursion. Aren't those 6V6's drawing very little screen current anyway like just a few milli-amps and not even a watt of power ?

[PeaveyBandit]

Hi KB,

Yes I see what you mean they aren't drawing that much. When I assembled the amp and all the connections I just assumed since I would be running the amp equally as hard as a Deluxe and almost diming it all the time the screen resistors and such would be a good idea. Especially after getting the recommendation from so many people but I can see now it's possibly not an important change. I may just leave the amp as is though. At least for now because it's sounding like a million dollars and powerful as any Deluxe I've ever had Except for maybe a few DRRI's which were pretty powerful but didn't sound anywhere as good as this one. Boy this hotrodded Princeton beefed up to the Deluxe range was the best idea ever. I can see why people came up with this upgrade path. It's just a fantastic and very musical amp. The bass is also utterly amazing to be a 12" speaker in a pocket sized cab. That was the one thing I was sweating but the thing has a punchy bass that just nails you.

I actually have a number of resistors and parts on hand to persue further changes and improvements but this amp ran perfect from the first power up and seems to be bullet proof so far, and sound so great I'm just going to leave it alone for a while and enjoy it. AGain, I can't thank you all enough for all this advice and help. I've learned a lot and have a whole lot more to learn.

The Hoffman adjustable bias upgrade with the 10KL pot and resistor was one of the smartest mods. I think being able to dial in the bias was a really good thing. The tremelo started working wonderfully after that.

[Rick Erickson]

anyone know about the rick e mod effect on AX7 reliabilty due to higher plate voltage?

Higher is a relative term. It will be higher than it was, but probably still lower than most Fender amps. This was merely a suggestion of something to try that was pulled from my notes on a Princeton amp that I had once owned. It still had the stock 10" speaker in it as I recall - I was just trying to make it a great sounding rehearsal amp.

RE

[stringpro]

Hello PB
I see this post from 2008 and I'm considering a build of your Modded Princeton Reverb .
Sounds like what the Dr. ordered !
After pounding on it for this long , have you any more to add ?

[RC99]

I was also looking to do the same and was wondering if there are any updates, or even recent thoughts. Does it still sound like a Fender?

[eschertron]

PB's been gone from the forum for a few years, but the thread sounds great! If anyone has done this and has a completed schematic for the beast, lay it on us!

[65dlxrev]

I recommend building a PR with DR power supply (including PT, filter choke and cap board) and a DR OT. This allows for much improved grounding and hum reduction and the DR PT and OT are definite upgrades over the PR iron. The PR, like the Champ, et al., uses that "multi cap" can with a common ground which means built in ground loop issues. Using seperate PS filter caps gives you the ability to ground each section of the PS correctly. I suggest running the CT of the PT HV winding directly to the negative leg of the first filter cap and then running a second wire back from this negative terminal to the chassis next to the PT core (where the CT is directly grounded in the stock Fender layout). Run the ground from the PI ("C" voltage) stage filter cap negative terminal to (1) that same chassis ground and (2) to the circuit card ground for the PI section of the circuit (one wire for each of these connections). You can run the ground from the filter cap for the screen supply ("B" voltage) to the chassis ground or to the brass ground buss at the end below the "intensity" pot. Finally, ground the last filter cap (preamp supply, aka the "D" voltage) and the grounds from the preamp section at a commn node on the brass buss below the "volume" pot. Avoid using the backs of pots for grounding. You can ground the "reverb" pot with a wire to the low side of the "volume" pot and ground the 6.8K "middle" fixed resistor via a wire between the resistor leg and the low side of the "volume" pot and then run a wire from the pot to the grounding node on the buss. Ground the low side of the "intensity" pot and the ground side of the 100K resistor on the "speed" pot on the buss below these pots. This scheme should ensure quiet, hum free operation.

You will have to enlarge the chassis hole for the PT and install a cap board with cover. I recently built such a hybrid using a PR cabinet from Mojotone and an Eminence "Ragin' Cajun" 10" speaker. This amp runs real quiet, breaks up with Vol at about 3 and really overdrives smoothly/warmly at higher Vol levels. I also like the bias based tremolo and concertina PI of the PR circuit. Definitely "fendery" but different flavor than my DR.

Good luck with your build.